1. Field of the Invention
This invention describes a novel transparent styrene-base copolymer which has excellent heat resistance.
2. Description of the Prior Art:
Polystyrene has conventionally been employed as a moldable and fabricable material which has excellent transparency and mechanical strength. Its use at elevated temperatures however encounters problems since it is inferior in heat resistance. It is therefore desirable to improve its heat resistance.
If improved heat resistance is the sole object, this can be achieved to a certain extent by copolymerizing maleic anhydride with styrene (see, for example, Japanese Patent Publication No. 40970/1983). Styrene-maleic anhydride copolymers, however, have insufficient stability with their molding or fabrication capabilities. They involve problems because they undergo decomposition, gelation and the like when their molding or fabrication temperatures become higher. There is therefor a very large demand for further improvements.
U.S. Pat. No. 3,766,142 discloses a copolymer composed of 25-90 mole percent of acrylonitrile, 1-25 mole percent of an N-arylmaleimide and the remainder of an ethylenically unsaturated monomer. Where the ethylenically unsaturated monomer is an aromatic olefin, the molar ratio of the aromatic olefin to acrylonitrile is disclosed to fall within a range of 0 0.5-4. The above U.S. patent discloses, in Example 33, that a terpolymer capable of providing a transparent plaque was obtained by subjecting acrylonitrile, styrene and N-o-chlorophenylmaleimide to emulsion polymerization.
Copolymers obtained in accordance with the above U.S. patent are however still insufficient in transparency.
U.S. Pat. No. 3,676,404 discloses copolymers each of which is composed of 80-95 wt. percent of methyl methacrylate unit, 5-20 wt. percent of an N-arylmaleimide unit and 0-15 wt. percent of a unit derived from any other ethylenically unsaturated copolymerizable compound (e.g., styrene unit or the like). These copolymers are described as having transparency and strength and as being suitable especially for the molding of various articles which are exposed to relatively high environmental temperatures.
These copolymers are however still insufficient in transparency.